Two fundamentally different switching technologies are used to enable digital communications, i.e. circuit switched networks and packet switched networks. Hybrid circuit switched/packet switched networks also exist in which packets are transmitted using TDM frames.
The circuit switched network operates by establishing a dedicated connection or circuit between two or more switch nodes within the circuit switched network. The packet switched network, on the other hand, typically connects computers and establishes an asynchronous “virtual” channel between two or more nodes within the network. In a packet-switched network, a data set, such as a voice signal, is divided into small pieces called packets which are then multiplexed onto high-capacity connections for transmission. Network hardware delivers packets to specific destinations where the packets are reassembled into the original data set.
The present disclosure relates to improvements in circuit switched networks and hybrid circuit switched/packet switched networks. More particularly, one example of a circuit switched network is a public switched telephone network (PSTN) that is used for making telephone calls. For example, a telephone call causes a circuit to be established from an originating telephone through local switching offices across trunk lines, to a remote switching office and finally to the intended destination telephone. When the circuit is in place, the telephone call is guaranteed a data path for digitized or analog voice signals regardless of other network activity. Within the PSTN there existed a need to transmit multiple subscribers' calls upon the same transmission medium. To accomplish this, network designers developed and make use of a protocol referred to as time division multiplexing (TDM).
Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more signals or bit streams are transferred simultaneously as sub-channels in one communication channel, but are physically taking turns on the communication channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. After the last sub-channel, the cycle starts all over again. Time-division multiplexing is commonly used for circuit mode communication with a fixed number of channels and constant bandwidth per channel. Time-division multiplexing differs from statistical multiplexing, such as packet switching, in that the timeslots are returned in a fixed order and pre-allocated to the channels, rather than scheduled on a packet by packet basis. Time-division multiplexing takes frames of user data, such as voice signals, and multiplexes them into a TDM frame.
The circuit switched network usually includes multiple switch nodes which are arranged in a topology referred to in the art as a “shared mesh network”. Within the shared mesh network, user traffic can be transported between any two locations using predefined connections specifying particular links and/or switch nodes for conveying the user traffic.
The switch nodes are each provided with a control module. The control modules of the switch nodes function together to aid in the control and management of the circuit switched networks. The control modules can run a variety of protocols for conducting the control and management of the circuit switched networks. One prominent protocol is referred to in the art as “Generalized Multiprotocol Label Switching (GMPLS)”. In general, Generalized Multiprotocol Label Switching includes protection and recovery mechanisms which specifies predefined (1) working connections within a shared mesh network having multiple switch nodes and communication links for transmitting data between the switch nodes; and (2) protecting connections specifying a different group of switch nodes and/or communication links for transmitting data in the event that one or more of the working connections fail. In other words, when a working connection fails, the Generalized Multiprotocol Label Switching protocol automatically activates one of the protecting connections into a working connection for redirecting data within the shared mesh network.
However, the protection and recovery mechanisms defined in GMPLS have overlooked a number of issues when scaling to large optical shared mesh networks including a problem referred to herein as “misconnection”. Misconnection occurs when a single set of network resources are allocated to protect multiple user connections. When there are multiple simultaneous network failures, it is possible that some of the protecting connections will be “pre-empted”, which is the stopping of a lower priority protecting connection in favor of a higher priority protecting connection. The pre-emption of the lower priority protecting connection may temporarily cause a misdirection of the data flowing in the circuit switched network. Misconnections can also be caused by other conditions or events, such as miscommunications between a control module and an input or output interface of a switch node, mis-configuration of a switch node, messaging errors, latency in control messages, protocol deficiencies or unavailability of resources.
For example, an exemplary mesh network 2 is shown in FIG. 1, by way of example. In FIG. 1, the mesh network 2 includes switch nodes A, B, C, D, E, F, G, H, I, J and K. In this example, the mesh network 2 includes headend switch nodes A and K; tailend switch nodes D and H; and intermediate switch nodes B, C, E, F, G, I and J. The mesh network 2 also includes two working connections which are shown by single dashed lines 3a and 3b; and two protecting connections 4a and 4b that are shown by solid lines. Thus, the working connections are formed by the switch nodes {A, B, C, D}, {K, J, I, H}; and the protecting connections are formed by the switch nodes {A, E, F, G, D}, and {K, G, F, E, H}.
In this example, the links between E, F and G are shared by both protecting connections 4a and 4b. The working connections 3a and 3b and the protecting connections 4a and 4b can be established by the switch nodes A-K using GMPLS protocols prior to any network failure.
In this example, all of the working connections 3a and 3b, and the protecting connections 4a and 4b are bi-directional. The working connections 3a and 3b, as well as the protecting connections 4a and 4b are preferably composed of time-slots, and are switched at each hop.
To illustrate the “misconnection” condition, the protecting connection 4b has a higher priority than a priority of the protection connection 4a. Initially the headend switch node A detects a network failure of the working connection 3a on a link between switch nodes B and C. The headend switch node A activates the protecting connection 4a by sending control messages to the switch nodes E, F, G and D, and then switches traffic to the protecting connection 4a. Then, a network failure on a link between switch nodes I and J, triggers the headend switch node K to activate the protecting connection 4b, which has a higher priority than the protecting connection 4a. The headend switch node K activates the protecting connection 4b by sending control messages to the switch nodes G, F, E and H, and then immediately switches traffic to the protecting connection 4b. 
When the switch nodes G, F and E process the control message sent by the headend switch node K, the protecting connection 4b will preempt the protecting connection 4a. However, due to latency and processing delays it is possible for the traffic to reach the switch nodes E, F and G prior to the processing of the control messages preempting the protecting connection 4a. Consequently, traffic intended for tailend switch node H may be directed to the headend switch node A.
The common solution within circuit switched networks is for the headend and tailend switch nodes to wait for an explicit acknowledgement from all of the switch nodes within the protecting connection before switching user traffic. This is called end-to-end acknowledgment. In this example, headend switch node K would wait until it receives an acknowledgement from the switch nodes G, F E and H before the headend switch node K switched traffic onto the protecting connection 4b. However, this may result in an unacceptable protection delay in long-haul optical networks.
Thus there is a need to eliminate the problems associated with a misconnection while avoiding an unacceptable protection delay in circuit switched networks. The present disclosure enables network operators to activate protecting connections immediately after detecting a network failure without misconnections.